I seem to remember them talking about it at the time of Tora Bora operations , like they were using them, but I guess not. Or was it a smaller version? I know Gulf War I sped up its development and I thought it had been used then too.
Another book by a vet talks about watching a whole mountain disapearing, blown to bits.... or were these underground charges?
” whole mountain disappearing”
Thermobaric devices are frequently used for cave complex type things because they use up all the air. We have them in lots of sizes, MOAB is just the biggest one.
We also use bunker busters which are very massive designed to penetrate very deep before exploding. So yes, “underground explosions”